In machines for the production of paper, paper pulp and similar products, the fibrous web is dewatered partly by being squeezed between two press rollers. For economical reasons it is generally desirable to remove the majority of the moisture of the fibrous web in the press section. In the last decades, intensive research has led to entirely new types of felts and cloths designed for dewatering purposes in the press section of paper machines. Owing to these new products, it has become possible to meet increasing demands on dewatering efficiency. At the same time, the machine speeds have become higher, with the result that the demands on the runability and wear resistance of these products also have increased.
The older type of press felts which comprised a woven, conventional product which was felted and napped has practically disappeared from the market and is replaced by needled felts. As a rule, these needled felts have a basic structure in the form of a base weave. This base weave may also be replaced by yarn material in one direction only of the two felt directions. The base structure of the needled felt may also be totally void of yarn materials. Up-to-date needled press felts usually consist of one compressible part which is positioned closest to the fibrous web that is to be dewatered, and one less compressible part which is designed to receive and carry away the water that is squeezed out of the fibrous web. When a felt of this kind is used in a papermaking machine, the same area of the felt passes the press nip several times every minute and in doing so it is exposed to a cyclic compression which is exerted in the direction of the felt thickness. In addition, the felt is gradually permanently deformed until it is reduced to a state of reduced function. Felts of today's structure furthermore are liable to damage in case e.g. lumps of pulp pass through the press nip, as the compressible felt layer lacks the elastic properties necessary to take the deformation stresses that are generated under these circumstances. Damage of this kind to the press felt or to the batt thereof often occurs in the form of a razor-sharp slit in the crosswise direction of the felt, and the appearance of the slit suggests that the felt breaks on account of a very high, localized tensile stress that is exerted in the longitudinal felt direction. By increasing the thickness of the felt it is possible to increase the total compression without affecting the less compressible felt part. Although the result is that localized compression may be earier taken by the felt without damaging the cloth, it does, however, also bring about the disadvantage of reducing the openness and rendering water through-flow more difficult. The increased proportion of the batt included in the compressible part of the felt also increases the plastic deformation of the felt, which means an increase of the successive reduction of the permeability and at the same time increased risks of damage.